A technology for configuring a battery pack by connecting a plurality of rechargeable cells in series so as to realize a high-voltage battery has been put to practical use. In recent years, batteries of this type have been attracting attention in mounting on vehicles such as electric forklift trucks, hybrid vehicles, or electric vehicles. Some of the batteries of this type are configured so as to connect a plurality of battery packs in parallel to each other in order to stably supply large electric power to a load.
When a plurality of cells are charged in a state in which the plurality of cells are connected in series, voltages of the respective cells (or charging capacities of the respective cells) sometimes become non-uniform. When the battery above is mounted on a vehicle, discharging of the battery when a motor for traveling is driven and charging of the battery when the motor for traveling generates power are repeated, and the repetition of charging and discharging also sometimes causes the voltages of the respective cells to be non-uniform. The non-uniformity of the voltages of the respective cells may cause the deterioration of some of the cells to be promoted, and may cause efficiency of the entirety of the battery to be reduced. The non-uniformity of the voltages of the respective cells can be caused due to manufacturing variations or deterioration over time of the respective cells. In view of the foregoing, a voltage equalizer has been proposed for example that equalizes voltages of a plurality of cells by discharging cells having voltages higher than a target voltage from among the plurality of cells by using resistors (see, for example, Patent Document 1).